Two principal types of rotary drill bits are employed for rock drilling for oil wells, recovering core samples, and the like. One type of rotary rock drill is a drag bit. Some of these have steel or hard faced teeth, but primarily they are set diamond drills such as described in U.S. Pat. No. 3,174,564. Typically in a set diamond drill the face is coated over much of its area with a hard material in which are embedded or "set" numerous diamonds. The diamonds protrude from the surface of the matrix and when the drill is used they rub on the rock, abrading shallow tracks and cutting primarily by a combination of compressive and shearing action. Good flow of drilling mud adjacent such set diamonds is important for cooling to prevent damage to the diamonds from overheating.
Another type of bit, described below in greater detail, uses rolling cone cutters mounted on the body of the drill bit so as to rotate as the drill bit is rotated. Combinations of drag bits and rolling cone bits have been proposed. For example, U.S. Pat. No. 3,174,564 to E. A. Morlan for a "Combination Core Bit", has a cylindrical crown encrusted with set diamonds for cutting an annulus around a core. The set diamonds protrude from the matrix tiny distances in the conventional manner. A plurality of rolling cone cutters with carbide inserts are mounted in special recesses around the cylindrical crown for cutting an outer annulus of considerably greater area than the inner annulus cut by the diamonds. Also, U.S. Pat. No. 1,506,119 describes a combination rotary cutting/diamond bit.
Recently a new product has become available that permits a new type of rock bit. The product is a diamond cutter described in greater detail hereinafter. Broadly, the diamond cutter has a wafer or plate of diamond about 0.020 inch thick and about 0.520 inch in diameter bonded to a tungsten carbide slug. This product was developed by General Electric and is commercially available under their trademark COMPAX or STRATAPAC. Such diamond cutters are available with a circular 0.520 inch diameter diamond wafer or with half of such a wafer as a semicircle.
The carbide slug can be inserted in a drill bit body so that the diamond plate protrudes therefrom at the proper angle for cutting rock. The cutting action by these diamond cutters is by shearing the rock much in the manner of conventional machining with cutting tools rather than the grinding-like action of conventional set diamond drills. Instead of finely ground material, much of the cut rock emerges from the drilled hole as appreciable size chips, somewhat like these from a rolling cone cutter. A rock drill having such diamond cutters protruding from its face has been built by General Electric.
A rock bit having such diamond cutters and rolling cone cutters is described in my U.S. patent application Ser. No. 585,975, filed June 11, 1975, now U.S. Pat. No. 4,006,788. This application is incorporated herein by this reference.
The use of rolling cone cutters in drilling rock is a well-known and long-established art. A typical rock bit includes three rolling cutters, each having a generally conical configuration, and each occupying much of a separate 120.degree. sector above the bottom of the well bore. Each cone is equipped with a number of generally circular rows of inserts or cutting elements. Some cones have hardened steel teeth integral with the cone. Many cones have tungsten carbide inserts or other hard material forming the cutting elements. As the cone rotates, the work surface of the inserts of each row are applied sequentially in a circular path upon the bottom of the hole in the rock that is being drilled. As the rolling cone cutters roll on the bottom of the hole being drilled, the teeth or carbide inserts apply a high compressive load to the rock and fracture it. The cutting action in rolling cone cutters is typically by a combination of crushing and chipping.
There are several distinct shapes of tungsten carbide inserts which are standard in the industry for rolling cone cutters, such as the conical, the double cone, the semiprojectile, and the chisel crest. All of these insert shapes, however, are generally characterized in that they comprise a cylindrical base for mounting in a rolling cone cutter and an end converging to a work surface. The work surfaces are blunt-pointed with a somewhat wedge-shaped configuration, meaning that the first engagement with the surface of the rock is but a relatively small surface area, but when indentation into the surface of the rock has progressed, the width or thickness of the cutting element which then comes into contact with the rock is greater.
In operation, a rolling cone drill bit is attached to the lower end of a drill stem or drill string, and rotated about the longitudinal axis of the drill bit on the bottom of a bore hole. Thus, the rolling cone cutters are caused to rotate, and as weight is applied to the bit by the weight of the drill string, the tungsten carbide inserts of the cones crush, chip, gouge, and scrape the formation upon which the bit is rotated depending on the presence or absence of skew of the cone axis. The particles of rock formation thus dislodged are carried out of the bore hole by drilling fluid such as drilling mud which is pumped downwardly through the drill stem and rock bit, returning to the surface of the earth via the annular space between the drill string and the wall of the bore hole being drilled.
The tungsten carbide inserts along the periphery of a bit, that is, nearest the base of the cones, and which define the diameter of a hole being drilled are known as gage inserts. As the rolling cone cutters rotate, the gage inserts scrape against rock at the periphery of the hole being drilled to dislodge rock formation by compression and gouging. Of all the inserts of a rolling cone cutter, the gage inserts are most susceptible to wear because they undergo both abrasion and compression as they scrape against the periphery of a bore hole. Any appreciable amount of wear on the gage inserts is undesirable because this could result in an undersized bore hole. When a replacement drill bit is inserted toward the bottom of an undersize bore hole, the replacement bit can pinch against the undersized portion of the hole and experience undue gage surface and bearing wear in reaming the undergage hole, thereby compounding the problem.
Rock bits are often made with the nominal gage diameter being the smallest acceptable size and an overgage tolerance of about 1/32 to 3/64 inch. Thus, for example, a nominal 77/8 inch bit has a minimum gage diameter at the gage inserts of 7.875 inch and a maximum gage diameter of about 7 29/32 inch.
Excessive wear on gage inserts can occur even though gage inserts generally are made of tungsten carbide, either by itself or combined with other materials such as cobalt. The gage row inserts are subjected to compressive loads like the other inserts in the cone. They are also subjected to abrasion by rubbing on the hole wall. Therefore, the gage cutting elements tend to wear faster than other cutting elements, and thereby can be a limiting factor on the life of a drill bit. Excessive wear due to abrasion on the gage cutting elements can necessitate premature replacement of the drill bit. Replacement is a time-consuming and expensive process, especially in deep bore holes, since the entire drill string must be removed from the hole in order to change the bit. Also, gage tungsten carbide inserts in a rolling cone cutter can exhibit poor wear resistance when drilling through formations containing steam or hot water containing corrosive salts such as when drilling for sources of geothermal energy.
Therefore, there is a need for a drill bit which avoids the drilling of undergage bore holes, including when the drill bit is used to drill for sources of geothermal energy.